Update production NiFi flow without stop and lose data

[#1]

When NiFi flow runs in mission critical production environment, customer would concern about how to change NiFi flow logic but don't impact business.

One solution was implemented in customer production is as follow steps:

1. Add an extra non-logic processor, for example "UpdateAttribute", between ListenPort and main ProcessGroup

2. Updated NewProcessGroup, add into canvas

3. Stop "UpdateAttribute" process, make sure RunningProcessGroup consume all flowfiles in the previous queue. But client components can still send messages to Listen Port

4. After all queued flowfiles are processed by RunningProcessGroup, move the queue end connection to the NewProcessGroup

5. Start "UPdateAttribute" process and NewProcessGroup

6. At the end, remove the old RunningProcessGroup

Follow these steps, the production flow change is transparent for client components, and guarantee no data lost.